Introduction: Although dual-task walking deficits challenge ambulatory function in persons with Parkinson's disease (PwPD), ambulation measures that incorporate cognitive dual-task loads seem scarce. In its construct and instruction, the Six-Spot Step Test Cognitive (SSSTcog) ensures an equal focus on cognitive and motor tasks. The present study investigated the construct validity and test-retest reliability of the SSSTcog in PwPD. Methods: Seventy-eight PwPD were recruited from outpatient clinics. The SSSTcog was completed twice within the same day and again three to seven days later. In addition, the cognitive Timed “Up and Go” test (TUGcog) and the Mini-BESTest were also conducted on the last day. Reliability and validity were estimated using Bland-Altman statistics, the minimal difference (MD), Intraclass Correlation Coefficient (ICC), and Spearman's rank correlation coefficient (ρ). Results: The SSSTcog was found reliable (ICC: 0.84–0.89; MD: 23.7%–30.2%) and showed moderate construct validity to the TUGcog (ρ = 0.62, p < 0.001). Weak correlations to the Mini-BESTest (ρ = −0.33, p < 0.003) indicated low construct validity. Significantly (p < 0.001) higher dual-task costs were seen when performing the SSSTcog (77.6%) compared to the TUGcog (24.3%). Conclusions: In PwPD, the SSSTcog showed promising construct validity, acceptable to excellent reliability, making it a valid measure of functional mobility, including cognitive dual-tasking. Higher dual-task cost for the SSSTcog indicated actual cognitive-motor interference while performing the test.